1. Field of the Invention
The present invention relates generally to apparatus for producing low pressure partially-ionized gases, e.g., plasmas, and more particularly to the production of highly uniform plasmas useful for treating planar materials such as semiconductor wafers in low pressure processing equipment. Such treatment can include etching, deposition, cleaning, and ion implantation.
Plasma generation is useful in a variety of semiconductor fabrication processes including etching, deposition, ion implantation and the like. Plasmas are generally produced in a low pressure gas by accelerating naturally occurring free electrons in the gas to the gas ionization energy, typically between five and twenty electron volts. Collisions between these energetic electrons and the gas molecules occasionally cause a molecule to be ionized, releasing an additional free electron. Each additional free electron is also accelerated and can also ionize gas molecules. The resultant partially ionized gas is called a plasma.
2. Description of the Prior Art
Plasma producing methods are conventionally based on providing a gas tight chamber, gas pumps and flow control means for accurate control of gas flow and pressure for one or more process gases, and adding energy to free electrons in the gases so that these electrons are accelerated to the ionization energy of the gas molecules. Ionization of process gas molecules frees additional electrons that can also be accelerated to ionization energy.
In semiconductor wafer processing in a gas the process rate can be increased by increasing-the energy of the gas molecules. This increased energy can be in the form of molecule ionization, in a dissociation of the molecules into smaller molecules, in a driving of the molecular electrons into higher energy levels, and in other modes. Positively-charged ions can also be accelerated to increased kinetic energies that can be directionally controlled. Steady-state electric fields are conventionally used to control both the magnitude and the direction of ion kinetic energy.
The imparting of energy to free electrons in a gas is typically accomplished with electrical fields, varying magnetic fields, or both.
One traditional method used in semiconductor wafer processing for both Generating the plasma and accelerating the resulting ions involves placing an alternating current (AC) voltage between two electrodes inside a gas-tight chamber, with a wafer to be processed held on one of the electrodes. The applied AC voltage accelerates free electrons in the gas between the electrodes up to the ionization energy level of the process gas molecules, thereby generating a plasma. A direct current (DC) voltage between the plasma and the wafer held on the wafer electrode also results.
The free electrons in the plasma move easily from the plasma to the semiconductor wafer when the wafer electrode is at a maximum positive voltage, and do not move from the wafer, even when the wafer is at a negative voltage relative to the plasma. A rectifier is thus provided that is equivalent to a thermionic diode vacuum tube, where electron current flows from a heated cathode to a cooler plate, but not the reverse.
A single source of energy, from the AC voltage between the electrodes, controls both the number of ions generated (the ion flux, or ion density of the plasma) and the kinetic energy with which each ion strikes the wafer (the ion field). Some control over the ratio between the ion flux and the ion field can be achieved by varying the gas pressure, the alternating current power frequency, and the chamber configuration.
The narrower line widths required for modern semiconductor devices requires independent control over the ion flux and the ion field, and the capability of operating at low pressure, in the range of one to ten millitorr. A number of plasma generation systems have been developed to provide the required independent control of ion flux and ion field at low pressures. Several configurations use the electron cyclotron resonance (ECR) principle, wherein radio frequency power is applied to a gas and a constant magnetic field applied at right angles to the varying electric field of the radio frequency power. The constant magnetic field is adjusted so that the curvature of the electron path due to the constant magnetic field results in a circular electron path at the same frequency as the radio frequency power. A common combination of frequency and magnetic field is 2.45 Gigahertz and 875 gauss.
An ECR plasma source can provide a plasma with high ion flux at low pressure, with a low ion field (kinetic energy of the ions striking the wafer). The ion field can be increased and controlled by an independent alternating current voltage on the wafer electrode. However, ECR systems are expensive and difficult to adjust.
There are several configuration variations using a varying magnetic field passing through an electrically insulating cylindrical process gas chamber for plasma generation. These variations couple the electron acceleration energy through the insulating cylinder walls by different arrangements of varying magnetic fields. One advantage claimed to this configuration is that wafer damage can be reduced by generating the plasma distant from the wafer and flowing the energized gas from the plasma generation location to the wafer. This general approach is called downstream plasma generation. Current experience indicates little, if any, advantage in terms of wafer damage for downstream plasma generation, and a serious reduction in process rate.
The optimum configuration for generating a plasma for semiconductor wafer processing makes a high ion flux planar plasma adjacent to the wafer, with low ion field (ion kinetic energy), and provides an independent power source for increasing and controlling the ion field. In general, the goal is to cause a uniform flux of ions to strike the wafer at a right angle, with the flux and field independently controlled.
A planar plasma source is described in U.S. Pat. No. 4,948,458, METHOD AND APPARATUS FOR PRODUCING MAGNETICALLY-COUPLED PLANAR PLASMA, issued to the present inventor, John S. Ogle, and which uses a spiral planar coil to generate a varying magnetic field that passes through an insulating window and accelerates free electrons in the process gases in a circular path parallel to the turns of the coil. This configuration provides efficient plasma generation with low ion kinetic energy, but has a "dead" area corresponding to the center of the coil in which there is much less electron acceleration.
Another planar plasma source is described in U.S. Pat. No. 5,277,751, METHOD AND APPARATUS FOR PRODUCING LOW PRESSURE PLANAR PLASMA USING A COIL WITH ITS AXIS PARALLEL TO THE SURFACE OF A COUPLING WINDOW, also issued to the present inventor, john S. Ogle, which uses a coil with its axis parallel to the window. The forward magnetic field is inside the coil and does not contribute to plasma Generation. However, part of the return magnetic field passes through the window and couples energy to the free electrons in the process gases.